Laws and regulations for exhaust emission from motor vehicles have been drafted in many jurisdictions because of pollution and air quality, primarily in urban areas. These laws and regulations often consist of sets of requirements which define acceptable limits for exhaust emissions (emission standards) for motor vehicles equipped with combustion engines. For example, emission levels of nitrogen oxides (NOx), hydrocarbons (HC), carbon monoxide (CO) and particles are often regulated for most types of vehicles.
In order to meet such emission standards, the exhausts caused by the combustion in combustion engines are aftertreated (purified). By way of example, a so-called catalytic purification process may be used, which is why aftertreatment systems usually comprise a catalyst. Further, aftertreatment systems may alternatively or in combination with one or several catalysts comprise other components, such as one or several particulate filters.
FIG. 1 shows the combustion engine 101 of a motor vehicle 100, where the exhaust stream generated by the combustion is led via a turbocharger 220. The exhaust stream is subsequently led via a pipe 204 (indicated with arrows) to a particulate filter (Diesel Particulate Filter, DPF) 202 via a diesel oxidation catalyst (DOC) 205. Further, the aftertreatment system comprises an SCR catalyst 201 (Selective Catalytic Reduction, SCR), arranged downstream of the particulate filter 202, which uses ammonia (NH3), or a composition from which ammonia may be generated/formed, as an additive for the reduction of the quantity of nitrogen oxides NOx. The particulate filter 202 may alternatively be arranged downstream of the SCR catalyst 201. The diesel oxidation catalyst DOC 205 has several functions and uses the surplus of air which the engine process generally creates in the exhaust stream as a chemical reactor jointly with a precious metal coating in the diesel oxidation catalyst. The diesel oxidation catalyst is normally primarily used to oxidise remaining hydrocarbons and carbon monoxide in the exhaust stream into carbon dioxide, water and heat, and conversion of nitrogen monoxide into nitrogen dioxide.
In connection with combustion of fuel in the combustion engine's combustion chamber (cylinders), soot particles are formed. For this reason, the particulate filter is used to catch soot particles and thus functions so that the exhaust stream is led through a filter structure where soot particles are caught from the passing exhaust stream and stored in the particulate filter. The particulate filter is filled with soot as the vehicle is driven and sooner or later the filter must be emptied of soot, which is usually achieved with the help of so-called regeneration. The regeneration entails that the soot particles (mainly carbon particles) are converted into carbon dioxide and/or carbon monoxide in one or several chemical processes. Regeneration may occur in various ways and may for example occur with the help of so-called NO2-based regeneration, often also called passive regeneration, or through so-called oxygen (O2)-based regeneration, also called active regeneration.
In connection with passive regeneration, nitrogen oxide and carbon oxide are formed in a reaction between carbon and nitrogen dioxide according to e.g. equation 1:NO2+C=NO+CO  (1)
The passive regeneration, however, is heavily dependent on the availability of nitrogen dioxide. If the supply of nitrogen dioxide is reduced, the regeneration speed is also reduced.
The supply of nitrogen dioxide may e.g. be reduced if the formation of nitrogen dioxide is hampered, which may e.g. occur if one or several components in the aftertreatment system are contaminated by sulphur which normally occurs in at least some types of fuel, e.g. diesel. Competing chemical reactions also hamper the nitrogen dioxide transformation.
The advantage of passive regeneration is that desired reaction speeds and thus the speed at which the filter is emptied are achieved at a lower temperature. Typically, regeneration of the particulate filter during passive regeneration occurs at temperatures in the range of 200° C. to 500° C., although temperatures in the higher part of this interval are normally preferable. Notwithstanding this, compared to the significantly lower temperature interval in active regeneration, this thus constitutes a great advantage if e.g. an SCR catalyst is present, since there is no risk that such a high temperature level is achieved that there is a risk of damage to the SCR catalyst. Nevertheless, it is important that a relatively high temperature is achieved in order for an effective passive regeneration to take place.
In active regeneration, so-called oxygen (O2)-based regeneration, a chemical process takes place mainly according to equation 2:C+O2=CO2+heat  (2)
Thus, carbon plus oxygen are converted, in active regeneration, into carbon dioxide plus heat. This chemical reaction, however, is heavily temperature-dependent and requires relatively high filter temperatures in order for a significant reaction speed to arise at all. Typically, a minimum particulate filter temperature of 500° C. is required, but preferably the filter temperature should be even higher in order for regeneration to occur at the desired speed. The reaction speed for chemical reactions, e.g. the reactions according to equation 1 and 2 above, are also dependent on the concentration of the reactants. If, for example, the concentration of any reactant is low, the reaction speed will be low, and if a reactant is missing, no reaction at all will occur.
Often the maximum temperature which may be used in active regeneration is limited by tolerances for some of the components comprised in the aftertreatment system/exhaust system. For example, often the particulate filter 202 and/or (where applicable) a subsequent SCR catalyst have constructional limitations with regard to the maximum temperature to which they may be subjected. This entails that the active regeneration may have a maximum component temperature which is often undesirably low. Simultaneously, a very high minimum temperature is required in order for any usable reaction speed to arise at all. In active regeneration, the soot load is normally essentially totally burned in the particulate filter 202. This means that a total regeneration of the particulate filter is obtained, following which the soot level in the particulate filter is essentially 0%. Today it is increasingly common for vehicles to be equipped, in addition to a particulate filter 202, with an SCR catalyst 201, which is why active regeneration may entail problems in the form of overheating for the subsequent SCR catalyst treatment process.
It is therefore of utmost importance to be able to stop a rapid increase in temperature in the exhausts before the SCR catalyst. Such a rapidly rising temperature may e.g. be the result of a runaway oxidation in the particulate filer (DFP), which may be inhibited or stopped if the concentration of oxygen into the particulate filter is reduced to a low or zero level. As mentioned above, it is also important, however, that the temperature be regulated in other components in the exhaust system, for example in order to prevent or inhibit local or global excess temperature in the particulate filter (DPF), etc.
Depending on how a vehicle is driven, the concentration/fraction for the exhaust stream resulting from the combustion will vary. If the combustion engine is working hard, the exhaust stream will have a higher concentration/fraction of combustion products and lower concentrations/fractions of combustion reactants, and if the load on the combustion engine is relatively low, the exhaust stream's concentration/fraction will be essentially the reverse. If the vehicle is driven for a longer period of time in such a manner that the exhaust stream contains relatively high concentrations/fractions of undesired combustion products, e.g. sulphur oxides, a degradation of the function of the diesel oxidation catalyst 205 will occur because the reaction of the sulphur which is usually present in the fuel in various forms with the active coating of the diesel oxidation catalyst 205, usually comprising one or several precious metals or other applicable metals such as aluminium. These problems usually arise at low (150° C.) to medium (300° C.) temperatures. At temperatures below 150° C. to 250° C., for example, an SCR catalyst will not function well. On the other hand, if the vehicle is driven for a longer period of time in such a manner that the exhaust stream's temperature maintains relatively high temperatures, this means that active regeneration may take place at the desired speed. However, the temperature in the exhaust stream may not exceed a maximum permitted temperature so that heat sensitive components in the aftertreatment system are damaged as previously mentioned. It is then particularly important to ensure that the concentration of NOx is maintained at low levels and that the balance NO2/NOx is optimal.
The concentration C of a substance in a gas may be expressed according to the equation:
      C    =          N      V        ,where N is the number of molecules of a given substance and V is the volume, i.e. the number of molecules of a given substance in a given volume. The total concentration CTot, which increases in an ideal gas if the pressure has increased and the temperature is falling, is expressed by the general gas law as
            C      Tot        =                  N        Tot            V        ,where NTot is the total number of molecules. The fraction X of a substance is expressed as the relationship between the concentration C and the fraction X as: C=X·CTot. If no chemical reactions take place, the fraction which specifies the proportion of molecules in a volume which belongs to a certain substance is not changed unless additional molecules are mixed with the original volume. This may occur through e.g. diffusion and/or through remixing of gas elements through so-called turbulence. The new molecules which are mixed in may come from e.g. urea and/or diesel injected into the exhaust pipe, which may be evaporated or reacted. They may also come from previously stored substances which are released, e.g. condensed water which is dragged along with the exhaust stream and/or evaporated. Examples of substances in the exhaust system which may be regulated are: carbon monoxide (CO) and nitrogen oxide (NO) which react with e.g. oxygen into carbon dioxide (CO2) and nitrogen dioxide (NO2) respectively.